The grip that tires have on the ground on which they are running is one of the most important features from the point of view of the safety of the driver of a vehicle fitted with tires. It is also of key importance in determining the performance of the vehicle in sporty road driving: if its tires lose their ability to steer as a result of a lack of grip, the vehicle can no longer be steered.
Of course, a vehicle, even if intended for sporty use, has to be driven in variable weather conditions. It is therefore known practice for the tire to be provided with means that provide good grip on dry ground and on wet ground. In particular, it is possible to tailor at least part of the tread pattern to use on wet ground, for example by providing recesses able to drain away and/or to store water, or by increasing the number of tread pattern edge features able to cut through the film of water formed between the tread and the ground. It is also possible to vary the materials of which the tread is made, using rubber compositions more particularly suited to use on wet ground and/or on dry ground. A tread comprising the two types of rubber composition is able to achieve good grip under all circumstances. An example of such a tire is given in document EP 1 308 319.
Under sporty road driving conditions, the tires of a vehicle experience substantial transverse loadings when the vehicle fitted with the tires is cornering. During the corner, these transverse loadings cause, on the contact patch where each tire makes contact with the ground on which it is running, deformation resulting in a somewhat trapezoidal shape: the side of the contact patch furthest away from the centre of the bend lengthens, while the side of the contact patch closest to the centre of the bend shortens.
The “side of the contact patch furthest away from the centre of the bend” is the side via which the elements of the tread come into contact with the ground in the direction of the rate of drift of the centre of the wheel on which the tire is mounted. For this reason, it is sometimes known as (transverse) leading edge. The opposite side, that is to say the “side of the contact patch closest to the centre of the bend” is sometimes known as the (transverse) trailing edge.
This “trapezoidal” deformation alters both the load borne by the various ribs of the tread and the contribution that each makes to the transverse force developed by the tire. For a given load that one of the tires of the vehicle has to bear at a given cornering speed, the ribs that have become lengthened bear a greater share of the total load borne by the tire. The ribs which have shortened bear a correspondingly lower proportion of the total load borne by the tire. For a given transverse force, delivered by one of the tires at a given cornering speed, what this means is that the most heavily loaded ribs (which in general means those on the side furthest away from the centre of the bend) are those which also make the greatest contribution to the total transverse force.
Rubber compositions suited to use on wet ground are generally more fragile with respect to the very high thermal and mechanical stresses generated in the contact patch of a tire at severe cornering speed on a dry road surface. If the tread of the tire is provided with portions made of a rubber composition with better grip on dry ground and with portions made of a rubber composition with better grip on wet ground, then it is preferable to ensure that the rubber composition that has better grip on dry ground is placed on the side of the contact patch that is furthest away from the centre of the bend. Thus, even if the contact patch becomes trapezoidal, the tire will maintain good grip on dry ground, that is to say a good ability to develop a high transverse force. Further, because the ground contact pressures are higher on this same side of the contact patch (which is the furthest away from the centre of the bend), the drainage of the water with which the road surface is wetted is generally rather satisfactory in this part of the contact patch. Consequently, this region of the tread is particularly suited to the use of a rubber composition with better grip on dry ground. In other words, the tire, in this region, runs as if it were running on dry ground. There is therefore no need to make this part of the tread from a rubber composition that has better grip on wet ground and of which the performance on dry ground is inferior to that of a rubber composition that has better grip on dry ground. The “Pilot Sport 2” tire sold by Michelin is an example of a tire in the tread of which the rubber compositions are arranged in this manner.
In spite of the good performance offered by this tire in terms of grip, there is still an increasing need to improve the compromise between grip on dry ground and grip on wet ground of tires, and more particularly of tires designed for sporty road driving. This is why the applicant proposed, in its patent applications WO 2011/076680 A1 and WO 2012/175444 A1, to divide the tread into several axial zones and to distribute the rubber compositions with better grip on wet ground and the rubber compositions with better grip on dry ground shrewdly over these zones. Although these solutions made it possible to obtain a better compromise on dry ground and on wet ground, they are not however optimal in terms of grip on wet ground.